Suppression of metal-to-metal charge transfer quenching in Ce³⁺ and Eu³⁺ comprising garnets by core-shell structure

This work concerns a novel Eu³⁺ activated phosphor sensitized by Ce³⁺ in order to obtain absorption in the blue spectral range in combination with line emission in the red spectral range. It is well known that this approach suffers from metal-to-metal charge transfer (MMCT) quenching between Ce³⁺ and Eu³⁺. To reduce this quenching mechanism in Tb₃Al₅O₁₂ (TAG), the sensitizer and activator were spatially separated by creating core-shell particles. Various powder samples including core-shell and non-core-shell particles were prepared via precipitation synthesis and post-annealing steps. The phase purity of the synthesized particles was determined by X-ray powder diffraction. Scanning electron microscope images have been taken to study the particle morphology and shell formation. It turned out that nearly uniform shells could be achieved by precipitation synthesis with urea as precipitant. The photoluminescence properties were investigated by recording emission spectra as well as diffuse reflectance spectra. Furthermore, temperature dependent emission spectra and fluorescence lifetimes were measured to compare thermal quenching and decay behavior of the samples. With around 60%, the external quantum efficiencies (EQE) of TAG:Ce³⁺ and TAG:Ce³⁺@TAG:Eu³⁺ core-shell particles are almost identical. Since the EQE of non-core-shell TAG:Ce³⁺,Eu³⁺ is lower than 5%, it could be demonstrated that the core-shell approach significantly increases the EQE by spatial separation of Ce³⁺ and Eu³⁺.